/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)

  Stockfish is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  Stockfish is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "tt.h"

#include <cassert>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <thread>
#include <vector>

#include "misc.h"
#include "thread.h"
#include "uci.h"

namespace Stockfish {

TranspositionTable TT;  // Our global transposition table

// Populates the TTEntry with a new node's data, possibly
// overwriting an old position. Update is not atomic and can be racy.
void TTEntry::save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev) {

    // Preserve any existing move for the same position
    if (m || uint16_t(k) != key16)
        move16 = uint16_t(m);

    // Overwrite less valuable entries (cheapest checks first)
    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_OFFSET + 2 * pv > depth8 - 4)
    {
        assert(d > DEPTH_OFFSET);
        assert(d < 256 + DEPTH_OFFSET);

        key16     = uint16_t(k);
        depth8    = uint8_t(d - DEPTH_OFFSET);
        genBound8 = uint8_t(TT.generation8 | uint8_t(pv) << 2 | b);
        value16   = int16_t(v);
        eval16    = int16_t(ev);
    }
}


// Sets the size of the transposition table,
// measured in megabytes. Transposition table consists of a power of 2 number
// of clusters and each cluster consists of ClusterSize number of TTEntry.
void TranspositionTable::resize(size_t mbSize) {

    Threads.main()->wait_for_search_finished();

    aligned_large_pages_free(table);

    clusterCount = mbSize * 1024 * 1024 / sizeof(Cluster);

    table = static_cast<Cluster*>(aligned_large_pages_alloc(clusterCount * sizeof(Cluster)));
    if (!table)
    {
        std::cerr << "Failed to allocate " << mbSize << "MB for transposition table." << std::endl;
        exit(EXIT_FAILURE);
    }

    clear();
}


// Initializes the entire transposition table to zero,
// in a multi-threaded way.
void TranspositionTable::clear() {

    std::vector<std::thread> threads;

    for (size_t idx = 0; idx < size_t(Options["Threads"]); ++idx)
    {
        threads.emplace_back([this, idx]() {
            // Thread binding gives faster search on systems with a first-touch policy
            if (Options["Threads"] > 8)
                WinProcGroup::bindThisThread(idx);

            // Each thread will zero its part of the hash table
            const size_t stride = size_t(clusterCount / Options["Threads"]),
                         start  = size_t(stride * idx),
                         len =
                           idx != size_t(Options["Threads"]) - 1 ? stride : clusterCount - start;

            std::memset(&table[start], 0, len * sizeof(Cluster));
        });
    }

    for (std::thread& th : threads)
        th.join();
}


// Looks up the current position in the transposition
// table. It returns true and a pointer to the TTEntry if the position is found.
// Otherwise, it returns false and a pointer to an empty or least valuable TTEntry
// to be replaced later. The replace value of an entry is calculated as its depth
// minus 8 times its relative age. TTEntry t1 is considered more valuable than
// TTEntry t2 if its replace value is greater than that of t2.
TTEntry* TranspositionTable::probe(const Key key, bool& found) const {

    TTEntry* const tte   = first_entry(key);
    const uint16_t key16 = uint16_t(key);  // Use the low 16 bits as key inside the cluster

    for (int i = 0; i < ClusterSize; ++i)
        if (tte[i].key16 == key16 || !tte[i].depth8)
        {
            tte[i].genBound8 =
              uint8_t(generation8 | (tte[i].genBound8 & (GENERATION_DELTA - 1)));  // Refresh

            return found = bool(tte[i].depth8), &tte[i];
        }

    // Find an entry to be replaced according to the replacement strategy
    TTEntry* replace = tte;
    for (int i = 1; i < ClusterSize; ++i)
        // Due to our packed storage format for generation and its cyclic
        // nature we add GENERATION_CYCLE (256 is the modulus, plus what
        // is needed to keep the unrelated lowest n bits from affecting
        // the result) to calculate the entry age correctly even after
        // generation8 overflows into the next cycle.
        if (replace->depth8
              - ((GENERATION_CYCLE + generation8 - replace->genBound8) & GENERATION_MASK)
            > tte[i].depth8
                - ((GENERATION_CYCLE + generation8 - tte[i].genBound8) & GENERATION_MASK))
            replace = &tte[i];

    return found = false, replace;
}


// Eeturns an approximation of the hashtable
// occupation during a search. The hash is x permill full, as per UCI protocol.
int TranspositionTable::hashfull() const {

    int cnt = 0;
    for (int i = 0; i < 1000; ++i)
        for (int j = 0; j < ClusterSize; ++j)
            cnt += table[i].entry[j].depth8
                && (table[i].entry[j].genBound8 & GENERATION_MASK) == generation8;

    return cnt / ClusterSize;
}

}  // namespace Stockfish
